It depends on your interests really. During my nursing career I've worked in a few different areas.
Accident & Emergency (ER) is rewarding in that some people get fixed and go home immediately, others more seriously ill may need a lot of input and to know that what you did may have helped save a life is rewarding beyond measure.
Care of the Older person can be rewarding in a different way. It is the pure nursing care that makes a difference here. You can turn somebody's life around by providing skilled nursing alone. Knowing that you've helped improve somebody's quality of life is a great gift.
I've worked in Burns & Plastics. Helping to rebuild somebody after a burn or traumatic injury gives plenty of job satisfaction.
In fact, every speciality you can think of has the potential to offer reward to the nurse, it all depends on your area of interest.